Based on how a 20th lvl Warrior can have almost a 100% Crit Chance because of how one of their traits work, I'd say that there isn't a cap. (Their trait gives a bunch of untiered +Precision for using all Signets, which greatly imbalances their Lvl vs Stat ratio. Once they start to get higher in level, the ratio balances out and it's no longer insane by the time that they hit 80th.)

Having said that, I have remembered reading that there is a diminishing returns break point in the +Stats department, but I honestly don't remember how high a Stat has to be for it to kick in.